Multiple sclerosis (MS) is an inflammatory neurodegenerative disease of the central nervous system, characterized by myelin loss and degeneration of axons (see, Blakemore, et al., J. of Neuroimmunology, 98, 69-76, 1999). Activation and CNS infiltration of the peripheral immune system is typical in early stages of the disease, but can become less prevalent as disease progresses.
A hallmark of MS is loss of myelin, accompanied by the death of associated oligodendrocytes (see, Merrill, J. E. et al., Neuropathology and Applied Neurobiology, 25, 435-458, 1999). Myelin, which is produced by oligodendrocytes, ensheathes axons and dramatically increases conduction velocity of neural impulses while providing trophic support to the neuron. Myelin is thought to regenerate early in disease, as oligodendrocyte progenitor cells (OPCs) proliferate and generate new myelinating oligodendrocytes in response to demyelination events. As the disease progresses the regenerative capacity of the OPCs becomes less robust, and axons remain chronically demyelinated. Chronic demyelination is thought to underlie axon loss, as loss of trophic support combined with the metabolic stress of transmitting impulses along a demyelinated membrane can lead to a breakdown of axonal integrity and permanent damage to the demyelinated circuit. In addition, exposure of a demyelinated axon to an inflammatory milieu, including infiltrating immune cells and activated microglia cells, is also thought to produce permanent damage and axonal loss. Axon loss as a result of demyelination is thought to underlie long term disease progression and disability in MS patients (see, Compston, et al., The Lancet, Vol. 359, 1221-1231, 2002 and D. Kremer et al, Trends in Neurosciences, Vol. 39, No. 4, 246-263, 2016).
The loss of remyelinating capacity in MS is not well understood, but is thought to involve a block in the differentiation capacity of OPCs, or the absence of a necessary signal present in the cell environment of the demyelinating lesion or in the demyelinated axons (see, R. Franklin et al., Nature Reviews/Neuroscience, Vol. 9, 839-855, 2008). The OPC cell population is prevalent in MS patients, but fails to generate new myelin in response to demyelination. Thus, a compound that can promote differentiation and myelination of OPCs should function to restore this regenerative capacity and blunt or reverse the degenerative effects of MS (see, Stangel, M. et al., Progress in Neurobiology, 68, 361-376, 2002, Nalm, F. J. et al., Nature (Letter), published online 20 Apr. 2015, doi:10.1038/nature14335). Such an agent could both increase the function of neurons and provide trophic support to enhance their survival (see, Mei, F. et al. Nature Medicine, Vol. 20, No. 8, 954-961, 2014).
Leukodystrophies are degenerative white matter diseases characterized by dysmyelination or demyelination. Multiple genetic or metabolic disorders can lead to progressive white matter damage in pediatric or adult populations resulting in severe motor or cognitive deficits, mental retardation or death. A compound that can delay myelin damage or promote repair of demyelinated axons could significantly alter the course of leukodystrophies and improve their outcome. Such a compound could be also useful in combination with other therapies that can correct the disease-specific defect, metabolic, genetic or other, responsible for initiating or maintaining the disease in order to accelerate repair, restore function or prevent further damage.
Hypoxic-ischemic insults leading to reduced oxygenation and blood supply into the brain can cause severe damage to OPCs, and demyelination. Periventricular leukomalacia is a condition characterized by toxic death of OPCs in the periventricular region and leading to severe dysmyelination and demyelination. This pathology has been proposed as the root cause of cerebral palsy, a life-long debilitating CNS disorder characterized by various motor and/or cognitive deficits of variable intensity. A compound promoting differentiation of surviving OPCs and remyelination of damaged areas could be used for the treatment or prevention of cerebral palsy in vulnerable infant populations.
Current therapies for MS are immunomodulatory in nature and do not directly promote repair. In addition, some of these immunomodulatory agents can leave patients vulnerable to opportunistic infection or neoplasia. Thus, there remains a need for compounds, such as those of the present invention, that can promote differentiation and myelination of OPCs and lead to the repair of demyelinated axons. Such a compound could also be useful in combination with existing or experimental immunomodulating and other relevant therapies to treat MS and other neurological and demyelinating diseases.